I am not an expert but I've played around quite a bit with the autotile tool.
to set the bitmask for this specific tileset:
set tilesize in the inspector to 3x3 (i forget the name of what you actually click)
then click inside the peach areas.
obviously to get what you are shooting for you'll need to adjust to your own needs
I expect you will need more tiles, the set above uses 55
I'm not very strong in the math/logic of all this, getting to this number was all trial and error, it's possible/probable that I didn't do this optimally.
I had to mess around to figure out how to put images in a post.
The reason for the apparent duplication of the top right's block of four tiles is so I could have inside and outside corners.